Torque converters are apparatuses that transmit torque from the engine to the transmission via a working fluid therein, and mainly include a front cover to which torque is input from the engine, an impeller fixed to a transmission side of the front cover to constitute a fluid chamber therebetween, a turbine located opposite an engine side of the impeller to output the torque to the transmission, and a stator located between a radially inner portion of the impeller and a radially inner portion of the turbine to adjust the flow of the working fluid from the turbine to the impeller.
A lock-up device is located in a space between the turbine and the front cover, and directly transmits the torque from the front cover to the turbine by mechanically engaging the front cover with the turbine. The lock-up device includes, for example, a disc-like piston to be pressed against a friction surface of the front cover for engagement, and an elastically coupling mechanism to transmit the torque between the piston and the turbine.
In the lock-up device, the piston has a frictionally coupling portion at a radially outer portion that is frictionally engageable with the front cover. Furthermore, the piston divides a space between the front cover and the turbine into a first hydraulic chamber near the front cover and a second hydraulic chamber, and can move in the axial direction due to a differential pressure between the first hydraulic chamber and the second hydraulic chamber (Refer to Patent Document 1, for example.). Next, the operation of the lock-up device will be described. When torque is transmitted through the fluid, working fluid is supplied to the first hydraulic chamber from the center, i.e., the working fluid flows radially outward and flows into the fluid operating chamber of the torque converter. Accordingly, the piston is separated from the front cover in the axial direction, i.e., in a clutch disengagement state. Then, at the lock-up engagement operation, the hydraulic circuit is switched to exhaust the working fluid in the first hydraulic chamber radially inward. Accordingly, the hydraulic pressure in the first hydraulic chamber becomes lower than that in the second hydraulic chamber so that the piston approaches the front cover. As a result, the frictionally coupling portion of the piston is forcefully pressed against the front cover, i.e., in a clutch engagement state.
Patent Document 1: Japanese Unexamined Patent Publication H11-63151